Transthyretin (TTR) is a homotetramer protein present in the blood plasma and cerebral spinal fluid. TTR is implicated in the formation of amyloid aggregates and deposition of amyloid fibrils, causing several pathologies in humans. The wild-type form of TTR is involved in senile systemic amyloidosis (SSA) in elders, due to deposition of amyloid mostly in the heart tissue. More than a hundred TTR variants are associated with amyloid formation and deposition of amyloid fibrils in various tissues and therefore with several familial amyloidoses, including familial amyloid polyneuropathy (FAP) and familial amyloid cardiomyopathy (FAC). In all cases, TTR aggregation seems to cause neuronal and/or cellular dysfunction by mechanisms that are not yet fully elucidated.
TTR plays a critical role in modulating the deposition of beta amyloid (Abeta) in Alzheimer's Disease. In addition, TTR stability is a key factor in TTR-Abeta interactions, which is relevant for the pathogenesis of Alzheimer's Disease. It has also been shown that administration of the TTR stabilizer iododiflunisal (IDIF) to AD/TTR+/− mice resulted in decreased brain Abeta levels and deposition and in improved cognitive function associated with reduced AD-like neuropathology in that particular mouse model. See, e.g., Choi et al. J Neurosci. 2007 Jun. 27; 27(26):7006-10; Ribeiro et al. PLoS One. 2012; 7(9):e45368; and Ribeiro et al. J Alzheimers Dis. 2014; 39(2):357-70; the entire contents of each of which are incorporated herein by reference.
Amyloid formation by TTR involves a first step wherein the native TTR tetramer dissociates to monomers with low conformational stability and increased tendency for partial unfolding. This is followed by self-assembly of partially unfolded monomers to form cytotoxic, oligomeric intermediate species, and eventually amyloid fibrils. Thus, stabilization of the native tetrameric form of TTR is a valid approach to reduce amyloid formation and can be attained by the binding of small organic molecules to tetrameric TTR.
It has been shown that thyroxine (T4) and several non-steroidal anti-inflammatory drugs (NSAIDs) bind to one or the two equivalent, funnel-shaped thyroxine-binding sites in TTR with high affinity, stabilize the tetramer and thereby prevent in vitro amyloid fibril formation. However, the use of NSAIDs in long-term treatments of TTR-amyloidoses is hindered by their poor selectivity for TTR and adverse anti-inflammatory effects. In addition, the serum concentration of T4 is relatively low (0.1 micromolar) compared to that of TTR (3.6-7.2 micromolar). Because thyroid-binding globulin (TBG) has an order of magnitude higher affinity for T4, less than 1% of TTR has T4 bound to it in the human serum.